This present invention relates to methods and apparatus for controlling a computed tomography imaging system. More particularly, exemplary embodiments of the present invention are related to methods and apparatus for operating a computed tomography (CT) imaging system in conjunction with a system of conveyors. Such a system may be used for security screening or non-destructive testing.
X-ray based security systems are deployed world-wide in airports to detect explosives, weapons, contraband, or other prohibited items in carry-on and checked baggage. Currently, most carry-on inspection systems are line scanners, which produce a transmission image (also known as scan projection or SP image) of a bag. More advanced CT systems are often used for checked baggage inspection. In many such systems, bags are transported into and out of the scanner by means of a system of conveyor belts.
Line scan systems utilize a scan projection (SP) image for presentation to the operator. In line scan systems, scan projection images are created by moving an object under a fan beam of x-rays from a stationary x-ray source. X-ray intensities, after being attenuated by the object being scanned, are measured by an array of detectors. The x-ray intensity data is converted through a process called normalization so that each pixel represents approximately the total mass traversed by the ray.
In computed tomography (CT) imaging systems, an x-ray source projects a fan-shaped beam, which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at each detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated around the object to be imaged such that the angle at which the x-ray fan beam intersects the object constantly changes. A group of x-ray attenuation measurements (e.g., projection data), from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector about the object or patient being imaged. A cross sectional image of the scanned plane can be reconstructed using well-known tomographic reconstruction algorithms such as direct Fourrier or filtered back projection methods.
Many modern CT systems are helical scanners (also known as spiral scanners), in which the scanned object is continually moved while the projection data is being acquired. The path of the X-Ray source describes a helix with respect to the scanned object. Most helical scanners have multiple rows of detectors, and the x-ray fan is collimated into a cone to illuminate the entire array of detectors. The angle between the x-ray source and the first and last detector rows is referred to as the “cone angle”.
The entire volume scanned by the helical scanner can be reconstructed using well-known tomographic reconstruction algorithms such as direct Fourrier or filtered back projection methods, and more exact methods described by Feldkamp and Katsevich. The advantage of a helical scanner is that much more of the object is scanned with each rotation of the x-ray source, thereby decreasing the time required to acquire sufficient data to reconstruct the scanned volume. For medical scanners this means that the patient must remain motionless for less time; for luggage scanners this means that more bags can be scanned per hour.
A CT system for checked baggage or carry-on items may be receiving objects (i.e., luggage) that need to be scanned from a series of conveyor belts, each of which provides an input and an output to the CT system. Moreover, the CT system itself will also have an independent conveyor that operates in conjunction with the output and input conveyors. Most conveyor systems at airports have a series of conveyors, each of which is configured to start, stop, slow down, or accelerate in conjunction with specific requirements of perhaps another single segment of the conveyor. For example, and when an output belt or slide of a single segment of a system is overloaded with luggage that has not been removed that segment will be stopped until the “log jam” is removed. This segment can no longer accept another bag, so if there is a bag on the preceding conveyor, the preceding conveyor must also be stopped, then the conveyor before that one must be stopped, and so forth, until perhaps the entire conveyor system has been stopped. This process is known as a dieback. When the original log jam has been cleared, each conveyor will start in sequence when the conveyor after it is ready to receive a bag.
A CT scanning system and in particular, a helical scanning system is typically configured to utilize reconstruction algorithms that are based upon a constant velocity of the item being passed through a gantry opening of the CT scanner. In other words, reconstruction algorithms that are based upon constant belt velocities will know exactly how much of the object as passed through the scanner after the x-ray source has made one complete revolution about the gantry as the object has passed therethrough. For example, each successive view from a particular x-ray source position along the gantry will correspond to a regular or constant interval and thus, the control algorithm of the scanning system will be written accordingly. As used herein, view refers to a particular location of the x-ray source along a path of rotation, which surrounds an opening of the CT scanning system through which the object is passed and scanned.
Accordingly, and when the CT scanning system is used in a conveyor system that must decelerate and stop, and then restart, this will mean that views (as defined herein) are not taken at regular intervals in the direction of belt movement, and standard spiral interpolation as is known in the related arts cannot be used. It is impractical to back up and restart a belt in a system of conveyors unless additional queuing conveyors are provided on both ends of the scanner. In addition, and if the conveyor belt is stopped for any length of time, a large amount of data will be acquired (e.g., the x-ray source will continue to rotate about the object and accumulate data). The data acquired after the belt stops and the CT source has made one complete revolution at the stopped position is redundant. The amount of redundant data acquired may become too large to save in an appropriately fast storage medium. Moreover, and if the conveyor belt is stopped for an extended time, the scanner may impart an unnecessary amount of radiation on the object being scanned. In order to ensure that the belt speed remains constant during the acquisition of a bag, CT scanners of the prior art must employ a conveyor system such that the entire bag can move through the CT gantry without stopping. Such a conveyor system requires longer or additional belt segments, increasing the overall size of the system. The conveyor system may also require greater spacing between bags, thereby reducing the number of bags that can be scanner per hour.
Accordingly, it is desirable to provide an apparatus and method for continuous data acquisition of an object being scanned when the conveyor belt runs at various speeds, or stops, as the object is being passed through the CT scanner.